In a digital camera, in response to the pressing of the release button, an image sensing device such as a CCD or CMOS sensor is exposed to an object image for a desired period of time. The resultant image signal representing a still image of one frame is converted into a digital signal. Predetermined processing such as YC processing is then performed for the digital signal to obtain an image signal in a predetermined form. Digital image signals representing sensed images are recorded on a semiconductor memory in correspondence with the respective images. The recorded image signals are read out as needed to be displayed or converted into printable signals. Alternatively, such signals are output to a display apparatus to be displayed.
Conventionally, a digital camera performs focus detection by using an output from an image sensing device. In this case, focus detection is performed by a contrast detection scheme. In focus adjustment based on the contrast detection scheme, the sharpness of an object image formed by an image sensing optical system is obtained by evaluating an output from the image sensing device by a predetermined function, and the position of the image sensing optical system on the optical axis is adjusted such that the function value takes an extreme value.
Evaluation functions include, for example, a function that adds the absolute values of the differences between adjacent luminance signals within a focus detection area, a function that adds the squared differences between adjacent luminance signals within a focus detection area, and a function that similarly processes the differences between adjacent signals of R, G, and B image signals.
In general, in focus detection based on such a contrast detection scheme, since evaluation function values are obtained while the position of an image sensing optical system on the optical axis is slightly moved, it requires a considerable time to perform focus adjustment until an in-focus state is obtained.
Japanese Patent Laid-Open No. 2001-215406 discloses an arrangement in which the photodetection surface of an image sensing device is stepped to realize high-speed direction determination in focus control. More specifically, a plurality of image signals are acquired by setting slightly different optical path lengths, and a focusing direction is determined on the basis of the acquired image signals. The photographing lens is then moved in the determined focusing direction up to the in-focus position. However, this scheme includes both pixels with short optical path lengths and pixels with long optical path lengths and hence is not suited to an apparatus aiming at obtaining high-quality images. Although the image quality improves as the optical path length differences between pixels with short optical path lengths and pixels with long optical path lengths are reduced, it becomes difficult to perform direction determination in focus control. It is therefore impossible to realize both high-speed focus control and high image quality.
On the other hand, as disclosed in U.S. Pat. No. 4,410,804, an image sensing apparatus incorporating a focus detection device based on a so-called phase difference detection scheme is also available, in which one or two pairs of light-receiving units are provided for each two-dimensional arranged microlens array, and the light-receiving units are projected onto the pupil of an image sensing optical system by using the microlenses, thereby dividing the pupil. In the phase difference detection scheme, an object image is formed by using two light beams passing through different portions of the pupil of the image sensing optical system, and the positional phase difference between the two objective images is detected on the basis of outputs from an image sensing device. This phase difference is then converted into a defocus amount of the image sensing optical system.
In focus detection based on the phase difference detection scheme, since a defocus amount can be obtained, the time required to obtain an in-focus state can be greatly shortened as compared with the contrast detection scheme.
In the image sensing device having the structure disclosed in U.S. Pat. No. 4,410,804, since one or two pairs of images formed by light beams passing through portions of the pupil of the image sensing optical system are output, these images have very low quality when evaluated in terms of image sensing performance for object images. The main cause for this is that in an out-of-focus background or foreground, unnatural image blur is caused by decentered light beams.
In order to eliminate such unnatural blur, there is available a method of combining one or two pairs of images formed by using portions of the pupil to obtain an image using all light beams passing through the pupil of the image sensing optical system in a pseudo manner. However, since signals are combined after they are output from the image sensing device, noise level is high due to amplifiers inside and outside the image sensing device. It is therefore difficult to obtain a high-quality image with a high S/N ratio similar to that of an image obtained by an image sensing device designed to obtain an image using all light beams passing through the pupil of the image sensing optical system.
Japanese Patent Laid-Open No. 07-168125 discloses an arrangement including a liquid crystal spatial optical modulator having a plurality of pixels between the microlenses and the photodetection surface of an image sensing device and is designed to form a three-dimensional image. This arrangement includes a spatial optical modulator which blocks the optical path between a microlens and a light-receiving unit of the image sensing device, and a control circuit which selectively causes light to be transmitted through only a given segment of the spatial optical modulator, thereby making the spatial optical modulator function as a shutter.
This arrangement indicates that images based on light beams passing through portions of the pupil of the photographing lens and images formed by using all light beams can be switched. However, since light is absorbed by the spatial optical modulator constituted by a liquid crystal layer and a polarizer, only an object with sufficient brightness can be captured.
Japanese Patent Laid-Open No. 11-119089 discloses a technique of using a single image sensing device both for image sensing operation and focus detection by extending and retracting a re-imaging optical system in and from a mirror box in a single-lens reflex camera. According to this technique, although high-speed focus adjustment can be performed by the phase difference detection scheme, a large retraction space and large energy are required to allow the large-size re-imaging optical system to move at high speed, resulting in an increase in the size of image sensing equipment.